Solutions having low pH and/or high concentrations of metal chlorides are produced as waste products in mining and other industrial processes or are present in water or waste water requiring treatment or are formed in the environment and cause salinisation of an environment.
Typically mineral processing and other industrial processes produce waste solutions that are heavily loaded with a range of metal ions, are often highly saline and sometimes acidic. Most metal ions are soluble in aqueous solution at low pH and are therefore not readily precipitated to allow separation or removal from the solution. Such solutions have been considered waste products and the recovery of acid and/or removal of metal ions was not considered viable.
Several methods to produce hydrochloric acid (HCl) and a metal hydroxide from salt solutions have been developed. These processes often use electrolytic cells that have cation exchange membranes which are susceptible to metal ion fouling. Other processes require significant purification of the metal chloride containing solution or complex apparatus and often produce only low concentrations of HCl. These processes are not practical or efficacious enough to be a commercially viable means of remediating waste water or producing high quality concentrated HCl.
A known process for producing sulfuric acid electrolytically using an anion exchange membrane is known (WO2010/083555). However, this process is unsuccessful with chloride ions because the chloride ions react at the anode to produce chlorine rather than hydrogen ions to produce HCl. Although attempts to prevent chloride oxidation at the anode have been investigated, including use of additional membranes to prevent chloride transport to the anode (K. Scott, Electrical Processes for Clean Technology, Royal Society of Chemistry, 1995) and use of catalytic anodes aimed at preventing chloride oxidation (D. Pletcher and F. C. Walsh, Industrial Electrochemistry, 2nd Edition, Springer, 1990), such solutions have not been efficacious in producing commercially economic HCl.
There is a need for a process that can be used to treat solutions containing high concentrations of metal chlorides and that may also be acidic, to produce high purity concentrated HCl, metal hydroxide precipitates and clean water, that is easy to use, has minimal membrane fouling, is insensitive to other non-metal chloride components that may be present in the solution and is efficacious at producing high purity HCl and reuseable water.